The increasing importance of quantitative imaging, for example to assess response to cancer therapies, have brought to the forefront the need to accurately measure the amounts of radioactivity administered to patients. It is therefore commonly recommended that the activity meters, also known as radionuclide calibrators or dose calibrators used in nuclear medicine be calibrated in a traceable, accurate and manufacturer independent manner.
Calibration involves the comparison of a clinical instrument with a reference instrument, for example, at a standards laboratory, and the subsequent adjustment of the clinical instrument based on the outcome of that comparison. This intercomparison can be achieved with relative ease for long-lived sources having sufficient half-life for this purpose, and which may be shipped between centers.
However, the majority of isotopes administered to patients, especially in the important area of positron emission tomography (PET) imaging, are of the short lived variety. Half-lives are measured in minutes and hours rather than weeks or years. Long-lived surrogate sources with decay schemes similar to a clinically used isotope are sometimes utilized for cross calibration, in conjunction with a suitable scaling factor. A notable example is the use of the 68Ge/68Ga surrogate, which has a half life of 270 days, for the clinically important isotope 18F which has a half-life of 110 minutes.
Using radioactive calibration samples has the obvious disadvantage that repeat measurements are impossible after the decay of the calibration source (e.g. a few weeks in the case of 131I). Moreover, the need to transport radioactive packages adds layers of administrative cost and complexity (and possibly shipping delays) to the process.
An alternative method for cross-calibration consists in the exchange of suitable instruments, which are calibrated at a location A and then shipped to a location B in order to transfer that calibration. This resembles the methodology utilized in external beam radiation therapy, where reference instruments (ion chamber, electrometer, cable), rather than radiation devices, are shipped and overall cross calibration with uncertainties of about 0.9% can be achieved. When applied to nuclear medicine, such cross-calibration methodology, would, at each location, use the clinically available, locally prepared radionuclides.
One possible approach to implement this strategy would be to ship the dose calibrators themselves between locations, with a replacement instrument available at the clinical site. However, dose calibrators are bulky and their shielded installation make this a cumbersome, expensive procedure. Another approach uses film which is exposed at a first location, where calibration curves are established for a given isotope under reference conditions. The reference conditions are then replicated at a second location, and again a film exposed. The optical densities obtained at the second location are converted into an activity by means of the film calibration curve. However, a large number of decays (≈2×1013) are needed to expose the film, which typically might take 24 hours for 99mTc and may be difficult to achieve for shorter-lived sources such as 18F. Furthermore establishing a calibration curve might take several of such measurements and needs to be repeated for each radioisotope for which calibration is desired. Another disadvantage is that the handling of film would require a low-background environment for the duration of the film calibration, which might not always be the case in a busy nuclear medicine department, and long-term, low-background storage of film stock and the maintenance of film scanners may provide logistical challenges in a nuclear medicine department.
Therefore, there is a need in the art for a method and apparatus for activity cross-calibration of unsealed radionuclides which may mitigate some or all of the disadvantages of the prior art.